Ultrasonic imaging systems are known to the prior art for providing real-time, cross-sectional images of human cardiac, abdominal and peripheral vascular structure that are of substantial diagnostic value. While various types of image formats have been provided, one of the more useful image formats from a diagnostic standpoint is the two-dimensional sector image that comprises an image of those body tissues located within a substantial planar sector. To provide such a sector image, the ultrasonic imaging system includes a scanner, a scan converter, and a display. The great majority of scanners in use today provide a mechanical scan of the sector and include a hand-held scan head that is maintained in contact with the skin of the body during use. The scan head contains a rotor that supports at least one ultrasonic transducer. During rotation of the rotor, the transducer is enabled for ultrasonic energy transmission and reception while the transducer is traversing a desired sectorial angle. When enabled, the transducer is caused to transmit ultrasonic energy and receive returns of the transmitted ultrasonic energy at each of a plurality of incremental angular positions (or scan lines) in the sector. The output signal from the transducer, representing the returns of transmitted ultrasonic energy, is in analog form and is referenced to a polar coordinate system. In order that the output signal may be visually displayed, in real-time, by a conventional video display whose scan is based on a Cartesian coordinate system and whose scanning rate may differ from that of the scanner, the output signal is buffered and converted to Cartesian coordinates by the scan converter before being supplied to the display.
Now considering the structure of a typical prior art scan head in more detail, the scan head includes a housing that defines a cavity in which the rotor is supported for rotation. The cavity is filled with an acoustic coupling fluid (such as oil or water) and is closed by a thin acoustic window that is either attached to or integral with the scan head housing. A plurality of ultrasonic transducers are disposed at equal angular intervals about the periphery of the rotor so as to be capable of ultrasonic energy transmission and reception in directions transverse to the rotor axis of rotation. The ultrasonic energy passes through the acoustic coupling fluid, the acoustic window, and an acoustic jelly that covers the acoustic window and that contacts the skin of the body. Provision is made for coupling electrical signals to and from the transducers on the rotor and for commutating these electrical signals so that each transducer is enabled only over the desired sector, e.g., during the time that the transducer is substantially aligned with the acoustic window. In order to drive the rotor, an electrical drive motor (typically a DC motor) is mounted within the housing. The motor shaft (or a shaft coupled thereto) extends into the cavity and is coupled to the rotor shaft, either directly or indirectly through a gear arrangement. To prevent leakage of the acoustic coupling fluid from the cavity into that portion of the housing containing the electrical drive motor, a seal (such as an O-ring seal) is disposed about the motor shaft. Finally, the rotor speed of rotation and angular position are sensed by an encoded member carried by the motor shaft and by an adjacent and stationary detector mounted in the housing.
Despite their widespread acceptance, the rotary ultrasonic scan heads known to the prior art have certain disadvantages. The leakage of acoustic coupling fluid from the cavity into the portion of the housing containing the electrical drive motor remains a problem and it has proven very difficult to provide an adequate, reliable, and long-lasting seal about the motor shaft to prevent such leakage. Although it is theoretically possible to provide a leakproof seal about the motor shaft, the resultant increase in friction makes such a seal unfeasible for hand-held scan heads in which the output torque available from the electrical drive motor is limited by the requirement that the electrical drive motor have a weight and size suitable for use in a hand-held housing. Although the prior art scan heads have been the subject of considerable development which has reduced their weight and size and increased their reliability, the scan heads remain expensive to manufacture due to the large number of separate mechanical components therein and often experience failure after an extensive period of use due to mechanical wear of these components. Due to the weight and size of the electrical drive motor and its drive train, the scan heads become tiring to manipulate after an extensive period of continuous use. Since the scan heads include an electrical drive motor to which electrical power is supplied, the scan heads oftentimes cannot be used in circumstances in which potential electrical or explosive hazards must be eliminated.
Yet another disadvantage results in the fact that the electrical drive motor and its drive train exhibit vibration during operation. Although this vibration may degrade the sector image developed by the scan head, it is most critical in the situation in which a fixed Doppler transducer is mounted on or integral with the scan head for the purpose of obtaining a duplex image containing both real-time position and velocity information. In this situation, it is essential that the scan head be vibrationless to optimize any signals from the Doppler transducer.
Due to their construction, the prior art scan heads cannot be modified or serviced by the user. As an example, the user may wish to change the operating characteristics of the scan head, such as the frequency, aperture, or focal length of the transducers. The prior art scan heads have not been designed to permit the user to disassemble the housing, replace the transducers and the rotor, and reassemble the housing so that the cavity is filled with acoustic coupling fluid and is devoid of air. If different operating characteristics of the scan head are desired, the user accordingly must either purchase a new scan head having the desired operating characteristics or return the existing scan head to the manufacturer for modification.